A typical axial flow gas turbine engine includes a compressor, a combustor and a turbine spaced sequentially about a longitudinal axis. Working fluid entering the compressor engages a plurality of arrays of rotating blades. This engagement adds energy to the fluid. Compressed working fluid exiting the compressor enters the combustor where it is mixed with fuel and ignited. The hot gases exit the combustor and flow into the turbine. The turbine includes another plurality of arrays of rotating blades that extract energy from the flowing hot gases.
Many steps are taken to maximize the efficiency of the gas turbine engine. In the turbine, each rotating turbine blade includes an airfoil that is shaped to engage the flowing gases and efficiently transfer energy between the gases and the turbine blade. Immediately upstream of each array of turbine blades is a stationary array of vanes. The vanes orient the flow to optimize the engagement of the flow with the downstream turbine blades. Radially inward of the airfoil and extending between adjacent airfoils is an inner platform. The inner platform defines a radially inner flow surface to block the hot gases from flowing radially inward and escaping around the airfoil. A corresponding radially outer flow surface is defined by a turbine shroud. The outer flow surface is in close radial proximity to the radially outer tips of the airfoils to minimize the amount of fluid that flows radially outward of the airfoils.
A typical turbine shroud is made up of a plurality of arcuate segments that are circumferentially spaced to form an annular structure. Each segment includes a substrate, a flow surface extending over the substrate, and means to retain the segment to the stator assembly outward of the array of blades. There are two commonly used types of retaining means. The first is a rail that extends along an axial edge of the segment and extends outward from the substrate. The rail includes a lip that engages a slot in the stator assembly. The other type is a plurality of hooks spaced along an axial edge of the segment and also extending outward from the substrate. The hooks also engage a slot in the stator assembly to retain the segments. One advantage of the hooks is the flexibility of the segment that results from not having a rail extending the length of the segment. In effect, the hooks are a segmented version of the rail, with the space between adjacent hooks providing additional flexibility. A drawback to the hooks is that, comparatively, the hooks have to be larger in cross-section to support the same load as the rail. This larger size limits the flexibility gain in using hooks rather than rails.
An additional function of the rails and hooks is to properly position the segment axially within the stator assembly. For this purpose, the axially facing surfaces of the hooks or rails is used as an axial position limiting surface. These positioning surfaces cooperate with mating surfaces within the stator structure to define the limits of the axial motion of the segment.
During operation of the gas turbine engine, the flow surfaces of the segments are exposed to the hot gases flowing through the turbine. To accommodate the extreme temperature present within the turbine, the segment may be coated with an insulating layer, such as a thermal barrier coating, and cooling fluid may be flowed over the radially outer surface of the segment. The cooling fluid is typically fluid drawn from the compressor and which bypasses the combustion process. In order to ensure that the cooling fluid flows into the flow path, rather than hot gases flowing outward, the cooling fluid is at a higher pressure than the hot gases flowing over the flow surface of the turbine shroud. The higher pressure cooling fluid loads the segments with a radially inward directed force that is reacted by the retaining means.
The segment has a hot side and a relatively cool side and therefore a thermal gradient across the segment develops. This thermal gradient encourages the arcuate segment to flatten out or bend in the opposite direction from its installed shape. This deflection places additional loading on the retaining means.
The retaining means, whether hooks or rails, must be of sufficient size to accommodate the bending stresses produced within the retaining means by the radially directed forces on the segment. Obviously, the larger the size of the hook or rail required, the greater the weight of the segment and the lower the flexibility of the segment. In addition, the retaining means may have to extend outward to provide a positioning surface for the segment. In instances where the segment is required to fit within a stator assembly having set dimensions, such as a segment being back fit into a pre-existing gas turbine engine, the required extension of the hooks or rails may increase the axial length of the hooks or rails and thereby amplify the bending stress in the hook as a result of the larger moment arm.
The above art notwithstanding, scientists and engineers under the direction of Applicant's Assignee are working to develop lightweight, flexible shroud segments for gas turbine engines.